Heat engines
g
• Heat engines are cyclic devices and that the working fluid of a heat engine return to its initial state at the end of each cycle.
heat engine return to its initial state at the end of each cycle. • Work is done by working fluid during one part of the cycle and on the working fluid
h
ki fl id during another part. (Deference between d i
h
(D f
b
these two equal to network delivered by the heat engine).
• To maximize efficiency: deliver most work and required least work. Internal combustion Engines: History,
History engine types and operation
of 2 & 4 stroke engines
• Maximum efficiency is given by ideal reversible cycle.
Dr. Primal Fernando
[email protected]
d@ d
lk
Ph: (081) 2393608
1
History of internal combustion (IC) engines
y
g
2
History of IC Engines
History of IC Engines
• Both
Both power generation and refrigeration are usually accomplished power generation and refrigeration are usually accomplished
by systems that operate on a thermodynamic cycle: power cycles and refrigeration cycles.
1860 Lenoir’s engine (a converted steam engine) combusted natural gas in a double acting piston, using electric ignition. Efficiency = 5% i
l
i i ii
Effi i
5%
• Power producing devises: engines
• Refrigeration producing devices: refrigerators, air‐conditioners and heat pumps.
• Steam engine ‐ 1700 (external combustion engines)
l
b
3
4
History ‐ continued
History Classification of Engines
Classification of Engines
• 1876
1876 Nikolaus Otto patented the 4 cycle engine, it used gaseous Nikolaus Otto patented the 4 cycle engine it used gaseous
fuel
• 1882 Gottlieb Daimler, an engineer for Daimler, left to work on his own engine His 1889 twin cylinder V was the first engine to
his own engine. His 1889 twin cylinder V was the first engine to be produced in quantities. Used liquid fuel and Venturi type carburetor, engine was named “Mercedes” after the daughter of his major distributor
his major distributor
• 1893 Rudolf Diesel built successful CI engine which was 26% efficient (double the efficiency of any other engine of its time) •
•
•
•
•
External vs Internal Combustion
Spark Ignition SI or Compression Ignition CI
Configuration
Valve Location
2 Stroke or 4 Stroke
2 Stroke or 4 Stroke
5
6
V Engine
g
Engine Configurations
Engine Configurations
In Line
(Automobile)
Horizontally
Opposed (Subaru)
Radial (Aircraft)
V
(Automobile)
Opposed Piston
(crankshafts geared
together)
7
8
Wankel Rotary Piston Engine
y
g
Rotary “Wankel”
Rotary Wankel Engine
Engine
Ref. Internal combustion engines and air pollution, E. F. Obert
9
10
Basic considerations in the analysis of power cycles
y
p
y
• Cycles encountered in actual devices y
are difficult to analyze because of the presence of complicating effects such as friction etc.
friction etc. • Consider a cycle that resembles the actual cycle closely but it made up t l
l l l b t it
d
totally of internally reversible process (ideal cycle) Thermal efficiency,  th 
11
Wnet
Qin
or
wnet
qin
12
Net work of the cycle
Idealizations and simplifications p
• Cycle does not involve any fi i
friction: no pressure drop in the d
i h
working fluid.
• Expansion and compression process: quasi equilibrium.
• Pipes connecting various components are well insulated.
• Neglecting changers in KE and PE
13
14
Air‐standard assumption
p
Carnot cycle y
• Gas power cycles (working fluid gas): spark ignition engines, diesel engines, conventional gas turbines, etc.
• All these engines energy is provided by burning a fuel within the system boundary.
• Working fluid (air) mainly contains nitrogen and hardly undergoing any chemical reactions in the combustion chamber and can be closely resembles to air at all times in the chamber. – Assumptions: working fluid as air, behaves as ideal gas, internally y
p
p
y
p
reversible cycle, combustion process replace by heat addition process by a external source, exhaust process replace by heat rejection process that re‐stores initial state of working fluid, specific heat values determines at room temperatures (call cold‐air‐standard assumptions)
assumptions).
• The Carnot cycle is the most efficient cycle that can be executed between heat a source
that can be executed between heat a source and a heat sink.  th,Carnot  1 
TL
TH
15
16
Reciprocating Engines
Parts of an engine
g
Top Dead Center (TDC)
p
: Upper most position
pp
p
Bottom Dead Center (BDC) : Lower most position
Exhaust
valve
Intake
valve
Stroke : Length of piston travel
TDC
Stroke
Bore
BDC
Bore : Diameter of the cylinder
Clearance Volume (Vc) : V where piston is at TDC
Displacement Volume (Vd) :Swept Volume (V
Displacement Volume (V
) :Swept Volume (Vmax‐Vmin)
Compression Ratio (rv) = (Vmax/Vmin) = (VBDC/VTDC)
Mean Effective Pressure (MEP) :
Wnet = (MEP) x (Displacement Volume)
Reciprocating Engine is INTERNAL COMBUSTION ENGINE, and is Classified into 2 types:
1.
Spark Ignition (SI): Gasoline Engine, Mixing air‐fuel outside cylinder, ignites by a spark plug (Auto cycle)
2
2.
Compression Ignition (CI): Diesel engine fuel is injected into the
Compression Ignition (CI): Diesel engine, fuel is injected into the cylinder, self ignited as a result of compression (Diesel cycle).
รศ.ดร.สมหมาย ปรี เปรม
17
Mean Effective Pressure, MEP Concept
18
Four Stroke Engine – spark ignition engine
Intake
Actual Processes
P
P
C
Compression
i
Power
Exhaust
Equivalent by MEP
Equivalent
Wnet
1. Intake Stroke piston moves from TDC to BDC,
drawing in fresh air-fuel mixture.
2. Compression Stroke piston moves from BDC to
TDC, compress air-fuel mixture.
3. Power Stroke piston at TDC, spark plug ignite
the air-fuel mixture. the combustion occur
very fast
f t that,
th t in
i theory,
th
the
th piston
i t still
till att
TDC. After that the piston is pushed to BDC.
4. Exhaust Stroke piston moves from BDC to TDC,
ppushes the combustion gases
g
out.
MEP
Wnet
vmin
TDC
vmax v
vmin
vmax
v
BDC
Wnet = (MEP) x (Displacement Volume)
= (MEP) x (Vmax-Vmin)
19
20
Actual and ideal cycle in spark ignition engine
i
Two Stroke Engine
Compression
Intake &
Exhaust
Power
1. Compression Stroke piston moves from BDC to TDC, compress air‐fuel mixture.
2. Power Stroke piston at TDC, spark plug p
p
p g
ignite the air‐fuel mixture. After the piston is pushed to BDC. Meanwhile, about half way, combustion gases are discharged and fresh air fuel mixture is
discharged and fresh air‐fuel mixture is drawing in .
g
g
y
2‐stroke engines generally less efficient than 4‐stroke engines since partial expulsion of unburned mixture with exhaust gas. It has higher power/weight ratio. 21
Air Standard Otto Cycle (Nikolaus A. Otto 1876)
22
T
Energy balance –
gy
Otto cycle (I)
y
Ideal cycle of spark ignition engine, comprises of 44 Process:
Process 1-2 Isentropic Compression (piston moves from BDC to TDC)
Process 2-3 v = constant, heat added (piston stays still, represents combustion)
Process 3-4 Isentropic expansion (piston moves from TDC to BDC gives POWER)
Process 4-1 v = constant, heat rejection (piston stays still, represents EXHAUST and INTAKE stroke)
Neglecting changes in KE and PE
2
(qin  qout )  ( win  wout )  u (kJ
k / kg
k )
There are only 2-stroke of all 4-processes,
P
T
3
Heat transfer to/from the system is under constant volume (no work)
qin  u 3  u 2  c v (T3  T2 )
wout
2
2
win
v2=v3
TDC
1
1
v1=v4
v
s1=s2
q out  u 4  u 1  cv (T4  T1 )
4
4
qout
s3=s4
4
qout
1
3
qin
3
qin
 th ,Otto
s
BDC
w
q
 net  1  out
qin
qin
Evaluate at room tem: called cold air standard assumption
standard assumption
T  T1
 1 4
 1
T3  T2
s1=s2
P
s
s3=s4
3
wout
T

T1  4  1 
T
 1

 T3

T2   1
T
 2

2
4
win
v2=v3
1
v1=v4
v
What is the different of Otto cycle from Carnot cycle, the most efficient cycle
23
24
T
Energy balance –
gy
Otto cycle (II)
y
T
 th ,Otto

T1  4  1 
w
q
 net  1  out  1  T4  T1  1   T1 
qin
qin
T3  T2
 T3

2
T2   1 
 T2

Processes 1‐2 and 3‐4 are isentropic and v
Processes
1‐2 and 3‐4 are isentropic and v2=v3
and v4=v1 (Pvk=constant)
T1  v 2

T2  v1



k 1
v
  3
 v4



k 1

qout
1
s1=s2
P
s
s3=s4
3
T4
T3
wout
V
V
v
r  max  1  1
Vmin V2 v 2
 th ,Otto  1 
 th ,Otto  1 
4
2
Compression ratio
Compression ratio
Thermal efficiency of a Otto cycle (I)
y
y
()
3
qin
1
4
win
v2=v3
1
v
v1=v4
r k 1
1
r
k 1
• High compression ratios: temperature of air/fuel mixture rises above auto ignition temperature (premature ignition)‐produces audible noise is k=1.4
called engine knock.
• Improvement of thermal efficiency was obtained utilizing higher compression ratios (up to 12) gasoline ble d ith tet aethyl lead (i
blend with tetraethyl lead (improving o i
octane rating) but it has been prohibited to use since the hazardous Octane rating = measure of fuel g
of lead to health
of lead to health. quality (measure of engines knock resistance)
25
Thermal efficiency of a Otto cycle (II)
y
y
( )
26
Compression Issues
p
Monatomic gas (He, Ar)
• Most
Most compression ratios are around 10:1, compression ratios are around 10:1,
meaning that the gas let into the cylinder is compressed to 1/10 times its original size.
air
CO2
k=1.2
• Efficiency is better with a higher compression ratio but only to the limits of the fuel quality.
ethane
Molecular weight of the working fluid increases
• Problems can occur during a cycle if there is:
Problems can occur during a cycle if there is:
– Lack of Compression caused from gasses leaking past the piston, a hole in the piston, or the intake or exhaust valves i t
h l i th i t
th i t k
h
t l
are not sealing properly.
– Lack of Spark caused by malfunctioning spark plugs, dirty spark plugs, mistimed firings, or bad connections between plugs and the battery.
Thermal efficiency of actual spark ignition efficiency of actual spark ignition
• Thermal
engine ~ 25‐30% 27
28
How Fuel is Handled
Chemical Energy of Gasoline
gy
• Structure of Gasoline
– Is mostly comprised of hydrocarbon molecules having Is mostly comprised of hydrocarbon molecules having
from six to ten carbon atoms.
• The
The chemical energy of one gallon of gasoline is, on the average, chemical energy of one gallon of gasoline is on the average
125,000 BTU per gallon (132×106 J per 3.8 L). – Octane
Octane is a measure of the resistance to detonation. The is a measure of the resistance to detonation The
octane number assigned to gasoline (87,89, 93, 100, 114, 120) represents the ratio of heptane, which easily detonates, to isooctane, which does not want to detonate
detonates, to isooctane, which does not want to detonate (better to say octane number above 100 as “performance number”. It is calculated by different way. Often itʹs done by pure extrapolation. ) . Eighty‐seven‐octane gasoline is yp
p
)
g y
g
gasoline that contains 87‐percent octane and 13‐percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). t
/h t
)
• Only about 25% of chemical energy in gasoline is converted to mechanical energy.
• Basically out of a one dollar gallon of gasoline, 75 cents is wasted.
29
30
Diesel cycle: The ideal cycle for compression ignition (CI) engine (Rudolph Diesel 1890)
ignition (CI) engine (Rudolph Diesel 1890)
Cylinder
y
Configurations
g
• Similar to spark ignition engine differing mainly in the method of initiating combustion.
fi ii i
b i
• In spark ignition (SI) engines (gasoline engines), air fuel mixture p
g
g
g
g
compressed below auto ignition temperature of the air/fuel mixture and combustion starts by firing spark plugs. Straight Configuration
V Configuration
Flat
Configuration
• In combustion ignition (CI) engines (diesel engines) air compressed above the auto ignition temperature of the air fuel mixture and then fuel inject into the air. Displacement refers to
the volume inside each
piston chamber.
chamber For
example: a 3.0 Liter
engine with 6 cylinders
will have 0.5 liters per
cylinder.
• SI engines has a carburetor and diesel engine has a fuel pump.
• The compression ratio of diesel engines typically higher (12 ‐24)
31
32
Energy balance – Diesel engine (I)
Diesel engine
g
(qin  qout )  ( win  wout )  u (kJ / kg )
• The fuel injection starts when the p
piston reaches to TDC.
q in  P2 (v 3  v 2 )  (u 3  u 2 )  h3  h2  c p (T3  T2 )
• Combustion process takes place over longer interval.
over longer interval.
q out  u 4  u 1  c v (T4  T1 )
• Because of this longer period the heat addition process can be
heat addition process can be approximated as constant pressure heat addition process.  th , Diesel
Di l 
wnet
q
 1  out
qin
qin
(T  T1 )
 1 4
 1
k (T3  T2 )
• Other parts are common for both SI and CI engines. T

T1  4  1
T
 1

 T3

kT2   1
T
 2

33
Otto vs. Diesel
Energy balance – Diesel engine (II)
 th , Diesel
q
w
(T  T )
 net  1  out  1  4 1  1 
k (T3  T2 )
qin
qin
T

T1  4  1
T

 1
 T3

kT2   1
T
 2

 th ,Otto  1 
1
r k 1
 th, Diesel  1 
1  rck  1 


r  k (rc  1) 
k 1
 th ,Otto   th , Diesel (when both cycles operate on the same compressio n ratio)
V3 v3

Define new quantity; cutoff ratio rc 
Define new quantity; cutoff ratio
V2 v 2
• Limiting value of rc=1; when efficiencies of both Otto and Diesel cycles are identical.
Utilizing definition of isentropic ideal‐gas relations
g
p
g
 th, Diesel
34
• Di
Diesel cycle operates much higher compression ratios, therefore l
l
h hi h
i
i
h f
thermal efficiency of Diesel engines are usually higher than SI engines (35 to 40%). 1  rk 1 

 1  k 1  c
r  k (rc  1) 
• Diesel engines burns fuels more completely than gasoline engines.
r is the compression ratio
Energy content of 1 gallon of diesel on average, 147,000 BTU per gallon (155×10
ll (155 106 J per 3.8 L).
J
3 8 L)
35
36
Dual cycle
y
• More
More realistic way to model:
realistic way to model:
Combination of heat transfer processes in gasoline and diesel cycles.
l
• The relative amount of heat transfer during each process can be adjusted to approximate actual cycle more closely.
37